How to texture crowded building
It depends on how detailed you want your textures.
Here are a few non accurate and fast methods:

Copy the material

You can try to determine 4-8 types of buildings (skyscrapers, living houses, hospitals, schools etc), make textures for them and just copy-pasted a resulting material to the other buildings, depending on their types.

Select all meshes, the last one set up with a random texture of a skyscraper.
Then go to materials tab in properties, then copy the texture to other selected meshes.
After that, for each mesh: tab to enter edit mode, then U -> Smart UV Project.



One big mesh

You can connect all meshes into one (A to select, Ctrl+J to join them), tab, then U -> Smart UV Project and make one big texture for them. After that, you can tune them after unwrapping to get better result (e.g. scale properly; set up roofs if you have them in your texture, etc). After that, you can separate meshes. In edit mode P -> by loose parts. And you will get several buildings with the same texture.

Can't provide a gif here.
The gif is too big, you can find it here.